1. Field of the Invention
The present invention relates to a method of producing a grain oriented silicon steel sheet having a particularly low iron loss, which can be advantageously used to form iron cores for transformers and other electrical equipment.
2. Description of the Related Art
Methods for lowering the iron loss of a grain oriented silicon steel sheet include the following: [1] increasing the silicon (Si ) content; [2] making fine secondary-recrystallized grains; [3] aligning the orientation of secondary recrystallization with &lt;1 0 0&gt;; [4] locally changing the deformation stress during cold rolling so as to improve the primary-recrystallized texture; and [5] reducing the impurity content.
Among these methods, method [1] (increasing the Si content) is not suitable for industrial production because such an increase greatly deteriorates the cold-rolling workability of the steel.
Various proposals have been made on method [2] (making fine secondary-recrystallized grains), particularly, on the art of designing cold rolling to achieve low iron loss. This art is in various forms, which are disclosed in various documents. One form utilizes the aging effect in which carbon (C) and nitrogen (N) are fixed by heat treatment in the dislocation previously introduced during cold rolling. Typical examples of this form include: adopting a temperature of 50.degree. to 350.degree. C. during rolling (Japanese Patent Publication No. 50-26493); achieving heat effect within a temperature range from 50.degree. to 350.degree. C. between cold rolling passes (Japanese Patent Publication Nos. 54-13846 and 56-3892); and adopting a combination of rapid cooling during hot-rolled steel sheet annealing and maintaining the steel sheet within a temperature range from 50.degree. to 500.degree. C. between passes. However, from the viewpoint of industrial production, these disclosed methods have many problems. For instance, cold rolling becomes difficult due to age hardening. Since the heat treatment process is added, the production efficiency is lowered. Further, after rolling, the surface roughness of the steel sheet greatly deteriorates, thereby making it impossible to improve magnetic properties significantly.
Aligning the secondary recrystallization orientation with &lt;1 0 0&gt; (method [3]) means increasing the magnetic flux density. At present, it is possible to carry out this method achieving a value approximately 97% of the theoretical value. Therefore, this method can be improved further only marginally, furthering iron-loss reduction only slightly.
Concerning method [4] (locally changing the deformation stress during cold rolling so as to improve the primary-recrystallized texture), Japanese Patent Laid-Open No. 54-71028 and Japanese Patent Publication No. 58-55211 disclose rolling with grooved rolls, and Japanese Patent Publication No. 58-33296 discloses cold rolling with dull rolls having a surface roughness of 0.20 to 2 .mu.m. These methods, however, have unresolved problems. Since the life of rolls is very short, this hinders production. The surface roughness of the steel sheet is so greatly deteriorated that, even when final-pass rolling is effected with smooth-surface rolls, the steel sheet tends to have poor surface roughness, thus making it impossible to improve magnetic properties Sufficiently.
Reducing the impurity content (method [5]) serves only slightly the purpose of lowering the iron loss. Impurities other than the inhibitor-forming component, such as phosphorus (P) and oxygen (O), aggravate the hysteresis loss. In order to avoid this problem, the current practice includes reducing the content of P and O to not more than approximately 30 ppm. Even if the P and O content is reduced below this level, the iron loss can be lowered only by a small margin from the currently obtainable value.